Description
Abstract: The two-body problem in general relativity can be described — in the post-Minkowskian expansion — purely in terms of on-shell amplitudes and their classical limit, without ever invoking a Lagrangian or equations of motion. However, beyond this perturbative regime, it is not known whether more complex aspects of binary dynamics — such as the merger phase or the influence of event horizons — can be captured on-shell. In this talk, I will argue that both the merger and the presence of an event horizon can indeed be described on-shell within a common framework based on mass-changing three-point amplitudes. The merger can be viewed as a fusion process once such amplitudes are introduced. Conversely, the presence of an event horizon can be described either as a decay process (Hawking radiation) or as a fusion process (classical wave absorption), following the same logic. This leads to a unified on-shell viewpoint in which black-hole mergers, gravitational-wave absorption, and Hawking radiation — seemingly distinct nonperturbative phenomena — reveal a surprising simplicity and common structure when formulated through on-shell methods. As an application, I will show how to derive conservation laws and compute memory waveforms associated with black-hole mergers, as well as how to describe the effects of Hawking radiation on the two-body problem using only gauge-invariant data and on-shell mass-changing amplitudes.
